The production of the coated papers having low grammage such as those mentioned above creates extremely high demands on the pulp properties, primarily because the pulp is required to have a high strength as well as a low roughness. In addition, these coated papers also require a low porosity, and it is also particularly important that they have a smooth surface structure.
These types of papers normally contain both chemical and mechanical pulps. The mechanical pulp component has traditionally comprised groundwood pulp. More recently, however, thermomechanical pulp (TMP) has been used as an alternative to groundwood. This has had limited success, however, because the concomitant energy consumption is relatively high compared with that in the manufacture of groundwood. Furthermore, in a number of instances the use of TMP has resulted in unevennesses in the surface structure of the paper, which in turn causes poor coating and printability characteristics. These problems could thus be avoided only by the paper manufacturer taking special measures to modify or eliminate the negative effects of the long fiber fraction present in the thermomechanical pulp. The presence of this long fiber fraction negatively affects the smoothness of the paper, because it causes poor formation of the paper and because it contains some long stiff fibers, as well as having poor binding strength. TMP for use in LWC-paper and the like is usually manufactured by refining in two or more steps, with subsequent screening and reject processing, bleaching and post-refining.